The present invention relates to a distributed feedback type semiconductor laser which is provided with a portion having periodic refractive index variations in the direction of light propagation in an active layer or a layer adjacent thereto and is caused to perform laser oscillation by injecting a current into the active layer portion.
The distributed feedback type semiconductor laser of this kind has, in an active layer or a layer adjacent thereto, a corrugation (a diffraction grating) providing periodic refractive index variations, and carries out a single wavelength operation, and hence possesses excellent characteristics as a light source for high quality optical fiber communication. Further, since the oscillation wavelength can also be adjusted as desired by changing the period of the corrugation in the fabrication process and since no cleaved planes are needed for a resonator unlike in conventional semiconductor lasers, this kind of semiconductor laser can also be applied as an integrated laser. However, if a lateral mode control mechanism is introduced into such a distributed feedback type semiconductor laser to reduce the stripe width down to about several .mu.m, the resulting laser output power inevitably becomes small. The reduction of the output is a problem in recently proposed, lateral mode controlled semiconductor lasers in which the resonator is usually formed by cleaved planes and, at present, the semiconductor laser output power is limited to several mW. An increase in the semiconductor laser output is one of the requirements for lengthening the repeater spacing in communication systems. One method that has been employed to meet such a requirement is to operate the semiconductor laser with a high injection current, but this method introduces a problem in the reliability of the laser because the injected current density and the power density of light at the laser resonator facets become high.